Recent years have seen the rapid development of photo-sensing charge coupled devices (CCD's) for electronic imaging of an actual scene or an image. Because of their many advantages (small size, low power, low cost, etc.), CCD's have become the imaging units of choice in many applications. CCD's are being used more and more in various imaging systems requiring very high resolution, full color balance, and wide dynamic range. By way of example, applications requiring very high performance of an electronic imaging system are found in the satellite imaging of fine features on the earth's surface from hundreds of miles in space, and in the near-perfect reproduction of an image from a frame of high resolution color film.
A CCD (change coupled device) photo-imaging unit typically has light sensing cells closely spaced apart in horizontal lines with the lines being closely spaced vertically. In one example of a CCD unit, termed a "linear CCD unit", there are thousands of such cells in each horizontal line and there are three such lines of cells very closely spaced vertically. Each line of cells reproduces a respective primary color (e.g., red, green and blue) of an image. A lengthwise portion of an image, such as a color photograph on a frame of 35 mm film, is then optically focused on the lines of cells. Each cell provides an electronic signal corresponding to the respective color intensity of a tiny portion, termed a pixel, of the image. The linear CCD unit is then scanned optically across the width of the 35 mm film frame to reproduce electronically the complete photographic color image on the film.
There are certain characteristics of a CCD photo-imaging unit which must be compensated for by the electronic analog signal circuitry which receives and processes the video signals produced by the CCD unit in order to obtain a truly high quality image. The electrical signal stored at each cell of the CCD unit is related to the intensity-time exposure of the light of an image incident on the area of that particular cell. The individual cells are made small (e.g., about 15 microns square) in order to obtain a large number of pixels along a line length (e.g., many thousands per length). Each cell has high impedance and the electrical signal obtainable from each cell (representing an image pixel) is relatively small (e.g., a volt or so). Therefore, even small levels of noise, such as thermal noise and switching transients within the CCD unit, become a significant factor which affects the quality of an image reproduced by the CCD unit.
An extensive discussion of CCD imaging units and some of the problems associated with them is given in an article by M. H. White, et al., entitled "Characterization of Surface Channel CCD Image Arrays at Low Light Levels", IEEE Journal of Solid State Circuits, Vol. SC-9, No. 1, February 1974, pages 1-14. This article describes the theory and operation of a CCD imaging unit and describes a method termed correlated double sampling (CDS) "to remove switching transients, eliminate the Nyquist noise associated with the reset switch/node capacitance combination, and suppress `1/f` surface-state noise contributions of a CCD unit". A schematic diagram of a CDS signal processor employing the method of correlated double sampling is shown in FIG. 5 on page 4 of this article.
As is well known, the individual cells of a CCD unit are adapted by means of respective color masks (filters) applied over the cells to respond to respective color components of an image. For example, certain cells are covered with red (R) masks, other cells by green (G) masks, and the remaining cells by blue (B) masks. Thus the R, G and B color components of an image are separately detected by respective ones of the R, G and B cells in a CCD unit. However, because of the differences in light transmittance of the green masks versus the red masks and the blue masks, the sensitivity of cells to green light is substantially greater than the sensitivity of the cells to red light or to blue light. The "green" cells generate (for a given total image brightness) substantially greater electrical output signals than do the "red" or the "blue" cells. It is necessary therefore to compensate for these differences in the R, G, and B signal outputs in order to obtain "amplitude balance" in an electronically reproduced image. In a linear CCD imaging unit, amplitude balance is easily accomplished by electronically controlling the "time exposures" (by means of control voltages) of the respective lines of R, G and B cells, as is well known in the art. When the R, G and B cells do not receive any light (i.e., total darkness), they still produce a small minimum "dark" signal voltage. Since the three lines of R, G and B cells in a linear CCD unit are all the same (only the color masks are different), the "dark" (no light) signals are substantially the same for all of the cells. Thus the linear CCD unit has a uniform "dark" signal characteristic of the R, G and B cells, along with amplitude balance of the respective signal color components R, G and B needed to reproduce a high resolution electronic color image.
The dynamic range of an analog image signal is conveniently expressed as a binary bit number. Thus an 8-bit number (with a decimal equivalent of 256) expresses the ability of a circuit to divide (digitize) the signal accurately into 256 parts. This in turn implies that noise and distortion contribute less than one part in 256 parts (about 1/2 percent) of the total image signal. By way of example, experimental high definition color television systems have a dynamic range of about 10-bits (the decimal equivalent of 1024), whereas color film typically has A dynamic range of 12 to 14-bits (with a decimal equivalent of 4,096 to 16,384). It is desirable therefore, in producing photographic-quality electronic color images using a CCD imaging unit to have an analog signal processor (ASP) for the output of a CCD imaging unit which has a dynamic range greater than 12-bits. Achieving such a wide dynamic range for an electronic imaging system of the kind described above implies that an analog signal processor (ASP) used to process the output signals from a CCD unit must itself cause less than about 0.014 signal distortion. By way of example, for a one volt CCD image signal, this means that the ASP should introduce less than about 100 microvolts total error into the high frequency analog image signals. This has been very difficult and expensive, if not impossible to do, with prior art signal processors.
Errors caused by an ASP which previously have been considered negligible when only an 8 to 10-bit dynamic range was acceptable, must now be either avoided or considerably reduced if an error level as low as 0.01% is to be achieved. In particular, several sources of error now become significant. These are as follows: minor timing differences in the actuation of sampling switches used in an ASP or ASPs to sample the high frequency CCD pixel image signals; signal-dependent charge-injection (causing "pedestal errors") occurring during turn-off of the sampling switches; and small thermally induced differential changes in the operating times and levels of the ASPS. The levels of these errors are so low (e.g., only a few millivolts) that it has previously been difficult to further reduce them without introducing other sources of error, or without adding greatly to the cost and complexity of the system.
A discussion of some of the considerations involved in designing a low noise track-and-hold amplifier (useful in an ASP such as may be employed in conjunction with a CCD imaging unit) is given in an article by M. Nayebi and B. A. Wooley, entitled "A 10-bit Video BICMOS Track-and-Hold Amplifier", IEEE Journal of solid State Circuits, Vol. 24, No. 6, December, 1989, pages 1507-1516. A doctoral Thesis written by Mr. Nayebi (Mohammed Reza Nayebi) at Stanford University, December, 1989, entitled. "Video BICMOS Sampling Systems" (available from Stanford University, Stanford, Calif., 94305), contains a related and much more extensive discussion.
It is highly desirable from the standpoint of low cost, small size, uniform quality, etc. to be able to implement an ASP as an integrated circuit (IC). The architecture of a circuit may impose limitations on the way in which it can be implemented as an IC. Conversely, there may be limitations on the performance of the circuit resulting from how it is implemented as an IC. The present invention provides a unique architecture for an ASP which is readily implemented as a single IC, and which achieves an overall dynamic range of greater than 12-bits in processing multiple signals from a linear CCD imaging unit, for example.